1. Field of Invention
The present invention relates to an active-matrix type electro-optical device and, more particularly, to an electro-optical device of the type that includes pixel switching thin-film transistors (TFTs) within a laminate structure on a substrate.
2. Description of Related Art
If incident light rays enter the channel region of a pixel switching TFT arranged for each pixel in a TFT active-matrix type electro-optical device, a current is generated through photoexcitation, thereby changing characteristics of the TFT. Particularly, in an electro-optical device being used as a light valve in a projector, it is important to block incident light entering the channel region of the TFT and the peripheral area thereof because of the high intensity of the incident light. This can be accomplished by a light shield layer, arranged on a counter substrate and defining the aperture area of each pixel, or a data line fabricated of a metal film such as Al (aluminum) and extending over the TFT blocks light entering the channel region and the peripheral area thereof. Japanese Unexamined Patent Application Publication No. 9-33944 discloses a technique which reduces light entering the channel region using a light shield layer fabricated of a-Si (amorphous silicon) having a large refractive index. Furthermore, a light shield layer fabricated of a refractory metal, for example, is arranged on the TFT array substrate in a position facing the pixel switching TFT (i.e., beneath the TFT). The light shield layer mounted beneath the TFT prevents a rear surface reflection from the TFT array substrate, or prevents projection light coming in from another electro-optical device and penetrating a prism from entering the TFT of the electro-optic device when a plurality of liquid crystal devices are combined to form an optical system via the prism or the like.
In the conventional art, each of the above light shield techniques suffer from one or more of the following problems. The light shield layers arranged on the counter substrate and the TFT array substrate are not sufficient to block light entering the channel region of the thin-film transistor and the peripheral area of the thin-film transistor. Additionally, the light shield layer and the channel region are substantially spaced from each other in terms of interlayer distance, in 3-dimensional terms sandwiching a liquid-crystal layer, thee electrodes, and interlayer insulators therebetween. This structure fails to block light obliquely entering between the light shield layer and the channel region. In a compact electro-optical device used as a light valve in a projector, incident light from a light source is condensed, and a component of incident light entering at an oblique angle with respect to the electro-optical device is too large to be neglected. Insufficient light blocking against the obliquely entering light presents practical problems.
For example, light enters into the electro-optical device through an area having no light shield layer and is reflected from the light shield layer and the internal surface of the data line (i.e., the surface of the data line facing the channel region). Such light may be further reflected from the internal surfaces of the light shield layer and the data line, thereby becoming multiple reflections. The reflected light and resulting multiple reflections may reach the channel region of the TFT. In the technique of using the data line as the light shield layer, the data line is formed in a stripe and extends in a direction perpendicular to the scanning line in a plan view, and an interlayer insulator laminated between the data line and the channel region must be thick enough to reduce the adverse effect of capacitive coupling therebetween to a negligible level. Accordingly, in this arrangement, it can be difficult to assure sufficient light blocking.
In accordance with the technique disclosed by Japanese Unexamined Patent Application Publication No. 9-33944, an a-Si layer is deposited on a gate electrode, and the lamination of a relatively thick interlayer insulator between the gate electrode and the a-Si layer is required to reduce the adverse effect of capacitive coupling therebetween. As a result, the additional lamination of the a-Si layer and the interlayer insulator enlarges the laminate structure of the device, and it is still difficult to fully block obliquely entering light and internal reflections. To satisfy current consumers"" demand for a higher quality display, high definition and high pixel pitch are required of the electro-optical device. As a higher definition display and a finer pixel pitch are introduced, the above-described light shield techniques have even more difficulty with assuring sufficient light blocking, and changes in the TFT transistor characteristics cause flickering, degrading the image quality of the electro-optical device.
To increase light resistance, the expansion of the formation area of the light shield layer has been contemplated. However, the expansion of the formation area of the light shield layer makes it difficult to heighten the aperture ratio of each pixel in an attempt to improve image brightness.
The light shield performance of the light shield layers formed on a counter substrate and a TFT array substrate is still not satisfactory. For example, the light shield layer having a light transmittance ratio of 0.1 to 0.01% is fabricated of Ti (titanium) or WSi (tungsten silicide). In the technique of using the data line as a light shield layer on the TFT array substrate, the light shield layer having a light transmittance ratio of 0.01% or so is typically fabricated of Al. Light from a light source of a projector is typically 10 M luxes or so. According to the study carried out by the inventors of this invention, the TFTs cause a photo-leakage current of 5E-11 [A] under light of 1000 luxes. As the electro-optical device has a higher definition and a finer pixel pitch to satisfy the consumers"" demand, a photo-leakage current is generated even by a low level of light in the above-reference light shields. Specifically, a change in the transistor characteristics visibly degrades an image on a screen.
A preventive step of thickening the light shield layer and the data line for higher light shield capability has also been contemplated. However, if these layers are thickened, the laminate structure of the substrate suffers from stress, and a diversity of problems are then expected including a warp of the substrate, an increase of processing time for film formation and etching steps.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 9-33944, the light transmittance ratio of an a-Si layer is substantially higher than that of Ti, WSi, Al, etc. In the electro-optical device incorporating higher definition and finer pixel pitch designs, a light shield layer fabricated of a-Si cannot block light from a light source in a projector, in particular.
The present invention has been developed in view of the above problems, and it is a first object of the present invention to provide an electro-optical device which presents excellent light resistance, features a high aperture ratio in each pixel, and displays a high-quality image.
It is another object of the present invention to provide an electro-optical device which presents excellent light resistance with a thickness increase of a light shield layer controlled and displays a high-quality image.
To achieve the above objects, a first electro-optical device of the present invention can include a pair of substrates, an electro-optical material interposed between the pair of substrates, a plurality of pixel electrodes arranged in a matrix on one of the substrates, thin-film transistors respectively electrically connected to the pixel electrodes, an upper light shield layer having a crossing portion above the thin-film transistor on the one substrate, a lower light shield layer having a crossing portion beneath the thin-film transistor on the one substrate and formed within the formation area of the upper light shield layer, and a junction of a channel region of the thin-film transistor which is formed within an area in which the crossing portion of the upper light shield layer and the crossing portion of the lower light shield layer overlap each other.
In the first electro-optical device of the present invention, the upper light shield layer having the crossing portion above the thin-film transistor on the one substrate defines the non-aperture area of each pixel. The upper light shield layer thus blocks light leakage, thereby effectively preventing a drop in contrast ratio. The upper light shield layer having the crossing portion is arranged above the thin-film transistor, the lower light shield layer having the crossing portion is arranged beneath the thin-film transistor, and in the image display area in a plan view, the formation area of the lower light shield layer is placed within the formation area of the upper light shield layer. At least the junction of the channel region of the thin-film transistor (the junction of the channel region with each of a source region and a drain region, each formed of one of an Nxe2x88x92 region, an N+ region, a Pxe2x88x92 region, a P+ region, etc.) is arranged in the crossing portion of the lower light shield layer.
Thus, in such applications as a projector that use intense light, the upper light shield layer is capable of shielding the thin-film transistor not only from a light component of the incident light entering at a right angle with respect to the substrate, but also from a light component entering at an oblique angle with respect to the substrate, to the thin-film transistor. The lower light layer is capable of blocking optical feedback including light reflected on the rear surface of the electro-optical device and light coming out of a light valve and passing through an optical synthesizing system when a plurality of electro-optical devices are combined to use another light valve as in a multi-panel projector application. Light rays entering the sides of the upper light shield layer may be reflected from the surface of the lower light shield layer facing the upper light shield layer, possibly becoming internal reflections and multiple reflections. With the upper light shield layer overlapping the lower light shield layer, these internal reflections and multiple reflections are effectively avoided.
Studies carried out by the inventors of this invention show that a photo-leakage current most likely occurs when light is incident on the junction of the channel region of the thin-film transistor. The junction of the channel region of the thin-film transistor is arranged within the crossing portion of the light shield layer which excellently blocks light rays entering at an oblique angle from either above or below or from the sides (in other words, the crossing portion of the light shield layer is least exposed to the incident light). In this way, less light is incident, and less likely a photo-leakage current occurs . In comparison with a conventional light shield layer arranged on a counter substrate, the TFT is light shielded by relatively closely laminated light shield layers from above and from below. In this arrangement, the light shield performance of the device is increased without unnecessarily expanding the formation area of the light shield layer (in other words, without unnecessarily narrowing the aperture area of each pixel).
A high aperture ratio of each pixel, and a high light resistance reduce degradation in performance of the thin-film transistor due to a photo-leakage current. An electro-optical device having a high contrast ratio and providing a high image quality is thus provided.
In one embodiment of the first electro-optical device of the present invention, the upper light shield layer is formed in a grid-like configuration to define a non-aperture area of each pixel, and the lower light shield layer is formed in a grid-like configuration.
In this embodiment, the non-aperture area of each pixel for a pixel electrode is defined by the crossing portion of the upper light shield layer arranged over the thin-film transistor. The lower light shield layer is narrower in width of vertical and horizontal segments of each grid thereof than the upper light shield layer (by one notch). This arrangement assures a higher light shield performance.
In the above embodiment, the upper light shield layer includes at least one electrode of a storage capacitor, one electrode of which is electrically connected to the pixel electrode, and a data line electrically connected to the thin-film transistor. Since the one capacitor electrode forming the storage capacitor and the data line are used as the upper light shield layer in this embodiment, the laminate structure can be simplified.
The upper light shield layer is configured in a grid with the data line and capacitive line mutually intersecting, and at least the junction of the channel region of the thin-film transistor is arranged within the crossing portion of the data line and the capacitive line. The junction of the channel region of the thin-film transistor is placed within the crossing portion of the data line and the capacitive line, because the crossing portion provides the most effective light shield performance to the image display area against light rays entering at an oblique angle from either above or below or from the sides . This arrangement makes it less likely that a photo-leakage current will occur at the thin-film transistor.
In the above embodiment, a semiconductor layer of the thin-film transistor is formed within an area where the region of the data line and the region of the lower light shield layer overlap each other. Since the entire semiconductor layer of the thin-film transistor is light shielded in this embodiment, occurrence of a photo-leakage current can be even more controlled in the thin-film transistor.
In the above embodiment, the upper light shield layer includes a plurality of first light shield layers extending in a first direction, an insulator layer formed on the first light shield layers, and a plurality of second light shield layers formed on the insulator layer and intersecting the first light shield layers extending in the first direction.
In this embodiment, the upper light shield layer is formed of the first light shield layers and the second light shield layers that mutually intersect the first light shield layers in a grid-like configuration. The junction of the channel region of each thin-film transistor is placed within the crossing portion of the first and second light shield layers. The junction of the channel region of the thin-film transistor is placed within the crossing portion of the first light shield layer and the second light shield layer, because that crossing portion provides the most effective light shield performance to the image display area against light rays entering at an oblique angle from either above or below or from the sides . This arrangement makes it less likely that a photo-leakage current will occur in the thin-film transistor.
In the above-referenced embodiment, the first light shield layer may be at least one electrode of a storage capacitor, one electrode of which is electrically connected to the pixel electrode, and the second light shield layer may be a data line electrically connected to the thin-film transistor. Since the one capacitor electrode forming the storage capacitor and the data line are used as the upper light shield layer in this embodiment, the laminate structure of the device can be advantageously simplified.
In another embodiment of the first electro-optical device of the present invention, at least one of the upper light shield layer and the lower light shield layer may be formed of the same single light shield layer. Since the light shield layer is formed of the same single light shield layer, the laminate structure of the device is simplified.
In the above-referenced embodiment, the same single light shield layer may include a plurality of light shield crossing portions, each arranged over the thin-film transistor.
To make it possible that a photo-leakage current will occur less in the thin-film transistor, light shielding at least the junction of the channel region of the thin-film transistor is effective. It is perfectly acceptable that each thin-film transistor is light shielded by one crossing portion.
In the above-reference embodiment, a scanning line, electrically connected to the thin-film transistor, may be formed within the region of the lower light shield layer. In this case, the scanning line may be fabricated of a silicon layer of polysilicon, amorphous silicon, or monocrystal silicon, or fabricated of polycide or silicide.
Further, in this arrangement, the scanning line fabricated of a silicon layer of polysilicon, amorphous silicon, or monocrystal silicon, or fabricated of polycide or silicide does not behave like a glass fiber, and thus effectively prevents incident light rays or optical feedback from being guided to the channel region of the thin-film transistor.
In the above-referenced embodiment, the scanning line may be formed within the region of the upper light shield layer. Since the scanning line is formed within the region of the upper light shield layer in this embodiment, the aperture ratio of each pixel is improved.
In yet another embodiment of the first electro-optical device of the present invention, a semiconductor layer of the thin-film transistor includes a channel, a region which is heavily doped with an impurity, a region which is lightly doped with an impurity and is arranged between the channel and the heavily doped region, and the lightly doped region is formed within an area where the crossing portion of the upper light shield layer and the crossing portion of the lower light shield layer overlap each other.
In this embodiment, occurrence of a photo-leakage current is controlled in the thin-film transistor having an LDD structure.
In yet another embodiment of the first electro-optical device of the present invention, the edge of the lower light shield layer in a cross section perpendicular to the one substrate recedes from the edge of the upper light shield layer corresponding to the edge of the lower light shield layer by 10 degrees or more with respect to a line normal to the substrate.
In this embodiment, the edge of the lower light shield layer in a cross section perpendicular to the one substrate recedes from the edge of the upper light shield layer corresponding to the edge of the lower light shield layer by 10 degrees or more. As long as the incident angle of light rays entering the sides of the upper light shield layer is 10 degrees or less with respect to a line normal to the substrate, internal reflections and multiple reflections, which can be caused when the incident light ray is reflected from the surface of the lower light shield layer facing the upper light shield layer, are prevented. Since obliquely incident light rays at an incident angle of 10 degrees or larger are almost non-existent in the electro-optical device used in a general-purpose projector, the recession of the edge of the lower light shield layer by 10 degrees or larger is important.
On the other hand, setting the angle of recession of the edge of the lower light shield layer not to exceed 10 degrees by a large angle appropriately controls internal reflections and multiple reflections which can be caused when optical feedback passing by the edge of the lower light shield layer is reflected from the surface of the upper light shield layer facing the lower light shield layer.
In yet another embodiment of the first electro-optical device of the present invention, a counter light shield layer is arranged on the other substrate, opposed to the one substrate, within the formation area of the upper light shield layer in a plan view.
In this embodiment, the counter substrate is provided with another light shield layer in the structure in which the an electro-optical material such as a liquid crystal is interposed between the substrate bearing the thin-film transistors and the counter substrate. Since another light shield layer is placed within the formation area of the upper light shield layer in a plan view, this light shield layer has no function of defining the aperture area of each pixel. However, this light shield layer blocks unwanted light incidence on the counter substrate, thereby preventing a temperature rise in the electro-optical device. The light shield layer on the counter substrate blocks unwanted incident light rays to some degree, thereby reducing a incident light component that can later become internal reflections and multiple reflections. Consequently, degradation in the thin-film transistor performance is reliably reduced.
To resolve the previously described problem, a projection-type display apparatus of the present invention can include a light source, a light valve including the first electro-optical device, a light guide member for guiding light generated by the light source to the light valve, and a projection optical member for projecting light modulated by the light valve. Since this embodiment prevents a photo-leakage current from occurring in the thin-film transistor in the electro-optical device, the projection-type display apparatus projects a high-quality image on a screen.
To resolve the previously described problem, a second electro-optical device of the present invention includes a pair of substrates, an electro-optical material interposed between the pair of substrates, a plurality of pixel electrodes arranged in a matrix on one of the substrates, thin-film transistors respectively electrically connected to the pixel electrodes, a light shield layer which is arranged over the plurality of thin-film transistors on the one substrate and is a laminate of a light absorption sublayer and a light shield sublayer, the light absorption sublayer being formed on the side of the light shield layer facing the thin-film transistors, and the light shield sublayer being formed on the side of the light shield layer opposite to the plurality of thin-film transistors, a data line electrically connected to the thin-film transistor and intersecting the light shield layer, and a junction of a channel region of the thin-film transistor which is formed within an area in which the light shield layer overlaps the data line.
In the second electro-optical device of the present invention, the data line and a main line portion of the light shield layer intersect each other above at least the channel region of the thin-film transistor connected to the pixel electrode. The electrically conductive data line having a light shield property and the light shield layer dually light shield the channel region of the thin-film transistor. If the electro-optical device is used so that the side on which the data line and the light shield layer are formed faces in the direction of incident light rays (such as projection light of a projector), the channel region of the thin-film transistor is dually blocked from the incident light rays. If the data line permitting light to be slightly transmitted (at a light transmittance ratio of 0.1%, for example) because of its thin thickness and the light shield layer permitting light to be slightly transmitted (at a light transmittance ratio of 0.1%, for example) because of its thin thickness are used together, high light shield performance (a light transmittance of 0.00001 to 0.000001%, for example) is achieved.
The light absorption sublayer of the light shield layer facing the thin-film transistor absorbs light rays passing by the thin-film transistor from the substrate and reaching the internal surface of the light shield layer (i.e., light rays reflected from the rear surface of the electro-optical device, and optical feedback coming out of another electro-optical device and passing through a light synthesizing system in a multi-panel projector which uses a plurality of electro-optical devices as light valves). The light shield layer is a multilayer formed of the light shield sublayer on the external side thereof (facing in a direction opposite to the thin-film transistor) and the light absorption sublayer on the internal side thereof (facing the thin-film transistor). The light shield sublayer is fabricated of a metal layer, such as of Al film or Cr film, having a high reflectance. The light shield layer enhances the light shield capability against light rays incident thereon, while reducing internal reflections generated within the light shield layer. As a result, light reaching the channel region of the thin-film transistor is reduced.
If a thick light shield layer is laminated on the substrate, the substrate can warp as a result of stress. Furthermore, if the size of surface irregularity and steps on the topmost layer serving as an underlayer of the pixel electrode is large in this type of electro-optical device, the electro-optical device suffers more from operational faults (such as orientation defects of the liquid crystal). If the laminate structure on the substrate is too thick, routing electrical connection to the pixel electrode becomes difficult. Accordingly, thickening the light shield layer to be embedded into the substrate and the entire laminate structure is basically undesirable. Required light shield performance is attained by laminating the two thin films, even if the two thin films individually are unable to assure sufficient light shield performance. Further, a layer other than a dedicated light shield layer is advantageously used as a light shield layer. Particularly, when the electro-optical device is used in projectors, which handle high intensity light, a high light shield capability is generally required. The dual light shielding construction in the crossing portion is effective.
The data line prevents light rays, incident at an oblique angle with respect to the substrate, from entering the channel region of the thin-film transistor. The light shield layer (extending in a direction perpendicular to the data line) prevents light rays, incident at an oblique angle with respect to the substrate, from entering the channel region of the thin-film transistor. Most of intense incident light rays enters at a right angle to the surface of the substrate, and the obliquely incident light rays have typically modest intensity, causing internal reflections and multiple reflections in the electro-optical device. The light shield performance level required to prevent the right-angle incident light rays is not so rigorous as that required to prevent the obliquely incident light rays. The obliquely incident light rays are effectively blocked by the data line and the light shield layer (even if it is a single layer).
As a result, even when high intensity light is used, a photo-leakage current due to the admission of light to the channel region of the transistor are prevented, and degradation in the transistor performance can be effectively avoided. The use of the light shield layer and the data line controls light leakage, thereby preventing a drop in contrast ratio. The aperture area of each pixel is also defined (therefore, the light shield layer conventionally arranged on the counter substrate may be dispensed with).
In comparison with a conventional light shield layer arranged on a counter substrate, the TFT is light shielded by relatively closely laminated light shield layers from above and from below. In this arrangement, the light shield performance of the device is improved without unnecessarily expanding the formation area of the light shield layer (in other words, without unnecessarily narrowing the aperture area of each pixel).
While the thickness increase of the light shield layer is controlled, light resistance is increased. It is less likely that a photo-leakage current will occur, thereby preventing degradation in the performance of the thin-film transistor. Accordingly, an electro-optical device having a high contrast ratio and providing a high image quality is thus provided.
In yet another embodiment of the second electro-optical device of the present invention, the light shield layer is arranged between the data line and the thin-film transistor. In this embodiment, the junction of the channel region of the thin-film transistor is covered with the light shield layer and then with the data line. High light shield performance is thus achieved. Since the light shield layer is interposed between the data line and the channel region, the adverse effect of capacitive coupling between the data line and the channel region can be reduced.
In still another embodiment of the second electro-optical device of the present invention, the data line is arranged between the light shield layer and the thin-film transistor. Since the junction of the channel region of the thin-film transistor is covered with the data layer and then with the light shield layer, high light shield performance can be achieved.
In the above-reference embodiment, a light absorption layer may be laminated on the data line on the side thereof facing the thin-film transistor. Since internal reflections and multiple reflections are absorbed by the light absorption layer of the data line in the electro-optical device, degradation in the thin-film transistor performance due to a photo-leakage current is reduced.
In yet another embodiment of the second electro-optical device of the present invention, the light shield layer may form a storage capacitor, one electrode of which is electrically connected to the pixel electrode.
The light shield layer has not only the function of light shielding, but also the function of serving as a pixel-potential capacitor electrode of the storage capacitor in this embodiment. The overall thickness of the light shield layer is controlled. This arrangement effectively precludes both a complex laminate structure and a complex manufacturing process of the laminate structure, which can result from a separate arrangement of a light shield layer and a storage capacitor.
In the above-referenced embodiment, the storage capacitor may include a capacitor electrode formed of the light absorption layer facing the thin-film transistor, a capacitor electrode formed of the light shield layer and opposed to the capacitor electrode of the light absorption layer, and a dielectric layer interposed between the two capacitor electrodes.
In this embodiment, the storage capacitor has not only the function of a capacitor, but also the function of light shielding and light absorption. This arrangement effectively precludes both a complex laminate structure and a complex manufacturing process of the laminate structure, which can result from a separate arrangement of a light shield layer and a storage capacitor.
In the above-referenced embodiment, a light absorption layer may be laminated on the capacitor electrode of the light shield layer on the surface of the capacitor electrode facing the thin-film transistor.
In this embodiment, the capacitor electrode of the light absorption layer and the light absorption layer formed on the capacitor electrode of the light shield layer absorb light of internal reflections and multiple reflections in the electro-optical device. Therefore, even if light leaks through the light shield layer of the capacitor electrode, the two light absorption layers reliably absorb light.
In yet another embodiment of the second electro-optical device of the present invention, the light shield layer may be connected to a constant-voltage line in a peripheral area surrounding a pixel display area in which the pixel electrode is arranged.
Since the light shield layers respectively facing the channel region and the data line are connected to a constant voltage in this embodiment, the channel region is free from the adverse effect resulting from variations in the voltage of the light shield layer. The light shield layer connected to the constant voltage in the peripheral area precludes the need for a complex laminate structure which can be required if the light shield layer is connected to the constant voltage within the image display area. Since the light shield layer also serves as fixed-potential capacitor electrode, a reliable storage capacitor is formed by connecting the light shield layer to the constant voltage. The constant voltage power source to which the light shield layer is connected may be a positive or negative constant voltage power source which feeds power to peripheral circuits for driving the thin-film transistor, or may be a constant voltage source for supplying power to a counter electrode on the counter substrate.
Another embodiment of the second electro-optical device of the present invention further includes a lower light shield layer arranged in a grid-like configuration beneath the plurality of thin-film transistors on the one substrate, wherein the lower light shield layer is formed within the formation area of the upper light shield layer, and covers a junction of the channel region of the thin-film transistor.
The lower light shield layer in this arrangement blocks optical feedback coming in from below the thin-film transistor. The thin-film transistor is thus light shielded from below and above. The lower light shield layer is formed of a single metal layer, an alloy layer, a metal silicide layer, a polysilicide layer, or a multilayer of these layer, each layer fabricated of at least a refractory metal selected from the group consisting of Ti, Cr, W, Ta, Mo, and Pb.
To resolve the previously described problem, a projection-type display apparatus of the present invention includes a light source, a light valve including the second electro-optical device, a light guide member for guiding light, generated by the light source, to the light valve, and a projection optical member for projecting light modulated by the light valve.
Since this embodiment prevents a photo-leakage current from occurring in the thin-film transistor in the electro-optical device, the display apparatus projects an high-quality image on a screen.
The thin-film transistor of the present invention may be of a top gate type in which the gate electrode, formed of part of the scanning line, is arranged over the channel region of the thin-film transistor, or may be of a bottom gate type in which the gate electrode, formed of part of the scanning line, is beneath the channel region of the thin-film transistor. The interlayer level of the pixel electrode may be above or below the scanning line on the substrate.